If using <script> tags with empty content, as when referencing a javascript file on the server, valid HTML would allow for the start tag to be closed with />.  For example,

<script language="javascript" type="text/javascript" src="scripts/test.js" />

However, the Visual Studio IDE interprets this as the start of a script section with no end tag.  The result is that no design view will be displayed.

To fix this problem, use a separate closing tag, as follows:

<script language="javascript" type="text/javascript" src="scripts/test.js" ></script>